Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging. The inks used in ink jet printers are generally classified as either dye-based or pigment-based.
A dye is a colorant which is molecularly dispersed or solvated by a carrier. The carrier can be a liquid or a solid at room temperature. A commonly used carrier is water or a mixture of water and organic co-solvents. Each individual dye molecule is surrounded by molecules of the carrier. In dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based ink jet inks, such inks still suffer from deficiencies such as low optical densities on plain paper and poor lightfastness. When water is used as the carrier, such inks also generally suffer from poor water fastness.
Water-based ink jet inks are generally made up of a water-soluble dye and/or a pigment dispersed or dissolved in water. Ink jet images printed with these inks have limited image fastness when exposed to humid conditions or water and usually results in significant loss of information which can render the image useless. In order to obtain water-resistance for such images, they usually need to be sprayed with a lacquer material or laminated with another material.
Water-resistant images are required to broaden the applicability of ink jet imaging technology to areas including textile printing, outdoor and indoor display markets, entertainment imaging, etc. However, many ink jet inks on the market are dye-based and use soluble colorants of various dyes. Such ink systems exhibit inherently poor water fastness and poor light stability especially when printed on ordinary bond paper. These dye-based printing systems are limited to the optical density these inks can provide.
In pigment-based inks, the colorant exists as discrete particles. These pigment particles are usually treated with addenda known as dispersants or stabilizers which serve to keep the pigment particles from agglomerating and settling out of the carrier. Water-based pigmented inks are prepared by incorporating the pigment in the continuous water phase by a milling and dispersing process. Pigmented inks require a water soluble dispersant in the pigment slurry during the milling process. Such a dispersant is necessary to produce a colloidally stable mixture and an ink that can be "jetted" reliably without clogging the print head nozzles.
Ink jet images prepared with pigmented colorants can offer near-archival light but have poor water fastness due to the water-soluble dispersant used to stabilize the dispersed colorants. Attempts to formulate these inks with addendum that impart water resistance (e.g., polymers, water repellants, etc.) are only marginally successful due to the presence of the water soluble dispersant(s) used to prepare and stabilize the pigment particles.